The present invention relates to extraction processes and more particularly to the use of liquid water at elevated temperatures and pressures as a solvent for the liquid-liquid extraction of a stream containing fatty acids and/or resin acids.
The fact that fatty acids can become completely soluble in liquid water at elevated temperatures has been known for at least 40 years (see Bailey's Industrial Oil and Fat Products, Volume 1, 4th Ed.).
Crude tall oil is a major by-product of the Kraft pulping process for making paper. The primary components are fatty acids, resin acids, and neutrals. In the prior art, resin acids are often referred to as rosin acids, and neutrals are typically indicated as unsaponifiables or non-saponifiables as they lack an acid group and therefore are not subject to saponification. The most valuable of the neutrals found in tall oil are the sterols, high molecular weight alcohols of biological importance.
The most widely used prior art method for recovering the acid components of tall oil is vacuum distillation. However, the major disadvantage of such process is that the sterols cannot be directly recovered. Most of the sterols present react with fatty and resin acids by esterification to form a heavy, low value residue known as pitch. Such reaction also decreases the quantity of fatty and resin acids recovered. Further, the unreacted sterols end up as impurities in the recovered resin acid streams. Northern and hardwood trees, which are now being used with greater frequency by the paper pulping industry, have a relatively high sterols content such that pitch formation and acid contamination is even greater when these raw woods are processed.
Thus, it is generally known in the prior art that sterols and other neutrals are best recovered by removal before distillation. Christenson et al U.S. Pat. No. 2,530,809 discloses a process for the fractionation of tall oil prior to distillation. Generally, tall oil, if not already present as such, is converted to a tall oil soap with fatty and resin acids present as soaps; neutrals (unsaponifiables), including sterols, are unchanged. The tall oil soap is mixed with a lower alcohol and subjected to an extraction with an organic phase which is immiscible with the soap solution but which acts as a solvent for the unsaponifiable neutrals. The neutral-free soaps are then converted to free fatty acids and free resin acids and are separated by conventional vacuum distillation. The neutrals are washed and stripped to eliminate the solvent.
Other patents which have employed this general scheme for the separation of tall oil into its various constituents include Hasselstrom et al U.S. Pat. No. 2,547,208, which discloses a method for refining tall oil soap employing ketones as a solvent for the undesirable neutrals; Chase et al U.S. Pat. No. 2,866,781, which discloses a method of separating non-acids from soap stocks in which an aqueous solution of soap is subjected to extraction with an ester solvent for the removal of unsaponifiable material; and Metchel et al U.S. Pat. No. 3,803,114, which discloses a process for purifying tall oil to produce unsaponifiable-free tall oil products wherein the unsaponifiables are extracted into a hydrocarbon phase. In Holmbom et al U.S. Pat. No. 3,965,085, a method for refining soaps using solvent extraction is disclosed in which the soap solution is first mixed with a low molecular weight ketone before the addition of a water-immiscible solvent such as hexane. The extracted soap phase is then distilled for removal of the ketone.
Cleary U.S. Pat. No. 4,495,094 discloses a process for separating fatty and resin acids from unsaponifiables in which the tall oil is not converted to a soap but is merely contacted with a solvent comprising an alcohol and water solution at room temperature which is selective for and extracts the fatty and/or resin acids. Kulkarni et al U.S. Pat. No 4,496,478 discloses a process for extracting unsaponifiables from fatty and rosin acids wherein an emulsion is formed with an organic solvent and an emulsifying liquid. A formation of three phases is effected by the application of centrifugal force. The three phases, an organic solvent phase containing the fatty acids, an emulsifying liquid phase, and a semi-solid sludge phase, are then separated.
Various patents are directed to the recovery of sterols and acids from the pitch produced during the distillation of tall oil. Generally, this pitch is treated by methods similar to those discussed above. It is converted to a soap and extracted with a solvent to remove the unsaponifiable matter. Christenson et al U.S. Pat. No. 2,530,810 discloses such a process wherein the soaps are dissolved in an alcohol prior to extraction by a hydrocarbon. The neutrals are then washed and stripped of the hydrocarbon solvent. Julian U.S. Pat. No. 3,840,570 discloses a process for preparing sterols from tall oil pitch wherein the pitch is dissolved in a solvent mixture of alcohol and hydrocarbon. Water, at temperatures ranging from 32.degree. F. to 212.degree. F., is then added to extract the acid soaps. The hydrocarbon phase, which contains the sterol esters, is saponified and the free sterols are recovered. In Lihtinen U.S. Pat. No. 3,926,936, the pitch is saponified at a temperature of 200.degree.-300.degree. C. The reaction product soaps are then acidified to produce an oil. The oil is distilled, and the distillate may be further refined by previously known fraction distillation processes. By such method, the sterols are dehydrated to form hydrocarbons, and the fatty and resin acids are recovered. Force U.S. Pat. No. 3,943,117 discloses a method for saponifying pitch in the presence of an amine catalyst to produce fatty and resin acid soaps. Harada et al U.S. Pat. No. 3,887,537 discloses a process for recovering fatty acids and rosin acids following pitch saponification by thin film evaporation.
Amer U.S. Pat. No 4,422,966 discloses a process for separating neutral compounds from tall oil soaps wherein the soap is contacted with a supercritical fluid solvent for the tall oil neutral compounds such that neutrals are extracted into the solvent. Preferably, the solvents employed by the Amer process are hydrocarbon gases which are exposed to supercritical conditions of temperature and pressure. Gases disclosed as suitable for the process include methane, ethane, propane, butane, ethylene, propylene and the like. One problem encountered in recent supercritical fluid extraction systems such as that of Amer has been the very low solubilities which many compounds of low volatility, particularly those which contain polar substituent groups, exhibit in supercritical gases. These low solubilities mean that the solvent recycle rates in a supercritical extraction process are very high such that the economics are less attractive.
Hughes U.S. Pat. No. 4,524,024 discloses a process of enhancing the recovery of fatty acids from tall oil pitch. Added to the generally known vacuum distillation process is an additional hydrolysis step. During this intermediate step, a pitch fraction is fed into a hydrolyzer at a pressure of from 40 kg/cm.sup.2 to 70 kg/cm.sup.2 where it is subjected to water having a temperature of from 260.degree. C. to 280.degree. C. During the hydrolysis step, free fatty acids are derived by hydrolytic splitting of the esterified fatty acids present in the pitch fraction. The entire hydrolysis reaction product is fed into the distillation process where the newly freed fatty acids are recovered.
Upon recovery of the neutrals by any of the prior art processes discussed above or by the process of the present invention, purification may be desired. U.S. Pat. Nos. 2,499,430 and 4,422,974 disclose methods for the recovery of sterols of high purity from neutrals.
Another problem to which the present invention is applicable is the purification of the deodorized distillate or sludge formed as a by-product during the deodorization of oils such as soybean oil, linseed oil, cottonseed oil, safflower oil, rice bran oil, corn oil and sunflower oil. Such distillate, like the tall oil discussed above, generally contains fatty acids and neutrals. The more valuable neutral components of the distillates are sterols and tocopherols (Vitamin E). Various methods for removing the fatty acids from the deodorizer distillate have been addressed by the prior art. For example, Takagi et al U.S. Pat. No. 4,454,329 discloses a process wherein the free fatty acids within the distillate are subjected to esterification by the addition of an alcohol. Sampathkumar U.S. Pat. No. 4,594,437 discloses a process whereby the free fatty acids are isolated by the formation of a urea complex.